transfer: readwrite improvements

- changed header/chunk/handler->readwrite prototypes to accept `buf`,
  `blen` and a `pconsumed` pointer. They now get the buffer to work on
  and report back how many bytes they consumed
- eliminated `k->str` in SingleRequest
- improved excess data handling to properly calculate with any body data
  left in the headerb buffer
- eliminated `k->badheader` enum to only be a bool

Closes #12283

lib/http.h

@@ -227,7 +227,8 @@ CURLcode Curl_http_size(struct Curl_easy *data);
 
 CURLcode Curl_http_readwrite_headers(struct Curl_easy *data,
                                      struct connectdata *conn,
-                                     ssize_t *nread,
+                                     const char *buf, size_t blen,
+                                     size_t *pconsumed,
                                      bool *stop_reading);
 
 /**